Such a steam cycle is known from, for example, DE 10226445 C1 or WO 2005/001248 A1. A cycle of the above mentioned kind is described there. Feed water is used as a working medium. Water is easy to handle, cheap and has good thermodynamic properties. The water is evaporated in a vaporizer. The steam is expanded in an expansion engine providing energy. After expansion the steam is condensed in a condenser and by means of a pump fed to a reservoir where it is available again for the cycle. The described power engine is used, for example, as an auxiliary power engine in motor vehicles.